The present invention relates to a control unit for driving an electric load, in particular an electric machine, having a first and a second voltage terminal in order to supply the control unit with electrical energy, a capacitor, which is connected between the voltage terminals, at least one half-bridge, which is connected between the voltage terminals, wherein the half-bridge has two controllable switches, between which there is formed a half-bridge tap for connecting the load, a safety circuit, which is designed to close one of the controllable switches of the half-bridge in the event of a fault, and a discharge circuit, which is designed to connect the voltage terminals electrically in the event of a fault.
The present invention further relates to a method for initiating a safety mode of a control unit for driving an electric load, in particular an electric machine, wherein the control unit has a first and a second voltage terminal in order to supply the control unit with electrical energy, a capacitor, which is connected between the voltage terminals, at least one half-bridge, which is connected between the voltage terminals, wherein the half-bridge has two controllable switches, between which there is formed a half-bridge tap for connecting the load. Said method comprises the steps: detecting a fault, closing a controllable switch of the half-bridge by means of a safety circuit and connecting the voltage terminals electrically by means of a discharge circuit.
Finally, the present invention relates to a drive train of a motor vehicle comprising an electric machine for providing the driving power, an energy supply unit for providing electrical energy and a control unit of the kind described above for driving the electric machine.
In the field of automotive drive technology, it is universally known to use an electric machine as the sole drive or jointly with a drive motor of another type (hybrid drive). Power electronics, which include a power inverter that converts the DC voltage/DC current of a battery situated on-board of the motor vehicle into an AC current, serve to drive such electrical machines in a motor vehicle.
Known power inverters comprise a plurality of switching elements, with which the individual phases of the electric machine are connected to a high supply voltage potential or to a low supply voltage potential. In so doing, two switching elements are each interconnected in series to a half-bridge, wherein a half-bridge tap is formed between the switching elements, at which tap the respective phase of the electrical machine is connected.
In the event of a fault, e.g. due to too high a battery current or too high a supply current, the power inverter is switched into a safe state in order to prevent possible damage to the electrical components. Essentially two different power-off methods are known from the prior art which are used alternatively. In the first method, individual or all of the controllable switches which are connected to the low supply voltage potential (low-side switches) are closed. This operating mode is also designated as the short-circuit mode. In the other power-off method the voltage terminals of the power inverter are connected to one another via an electrical resistor in order to produce a discharge of the intermediate circuit capacitor of the power inverter. Such a discharge circuit can be selectively implemented as a permanently energized discharge, which however permanently converts energy from the intermediate circuit capacitor into heat and therefore leads to a reduced degree of efficiency, or even implemented as a discharge circuit which can be switched on or switched off and which must be activated, for example, if an opening of the traction network connector is recognized.
The German patent application DE 10 2006 003 254 A1 discloses a method for operating an electric machine with a pulse width modulated inverter in the event of a fault, in which method the electric machine is initially switched into a disconnected mode and subsequently into a short-circuit mode.
A disadvantage of the known power-off methods is that the individual disconnections are activated using different triggering criteria and therefore require considerable technical complexity, high costs and large installation space.
It is therefore the aim of the present invention to provide an improved control unit for driving an electric load, which control unit can be switched into a safe state with less technical complexity. It is further the aim of the present invention to provide a corresponding method.